Dramatic CCTV has emerged showing the moment a doctor punches a patient in the head - who then collapses and later dies.

Russia opened an investigation on Saturday after a hospital doctor was captured on video punching a patient, knocking him to the floor and killing him instantly.

The shocking video of the attack in the southern city of Belgorod was aired repeatedly on state television and went viral on YouTube, while Russian media nicknamed the attacker "the boxer doctor."

The incident raised questions over a cover-up culture in state medicine, with investigators only announcing the probe several days later, after security camera footage was released online and shown on television.

In the video, the strongly-built doctor in medical clothing drags the barechested male patient from the examination table, asking him "why did you touch the nurse?" and pushes him out of the doorway.

When the patient returns, the doctor deals him a single blow to the face and a crash can be heard as the middle-aged man falls backwards onto the floor.

Meanwhile the doctor continues to scuffle with another man accompanying the patient, and it is only minutes later that the medics notice the patient lying motionless and attempt unsuccessfully to revive him.

Belgorod's Investigative Committee said in a statement released Saturday that the incident took place on December 29.

The doctor hit the patient in the face after he "kicked a nurse during a procedure," the investigators said.